Docs: handle versions when "instant loading" feature enabled (#12142)

Closes #12032

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- User's documentation (`docs`):
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Co-authored-by: Anthony <[email protected]>

Author: humitos

docs/user/intro/mkdocs.rst

@@ -158,6 +158,13 @@ To integrate the :ref:`flyout-menu:Addons flyout menu` version menu into your si
         </ul>
         </div>`;
 
+            // Check if we already added versions and remove them if so.
+            // This happens when using the "Instant loading" feature.
+            // See https://squidfunk.github.io/mkdocs-material/setup/setting-up-navigation/#instant-loading
+            const currentVersions = document.querySelector(".md-version");
+            if (currentVersions !== null) {
+              currentVersions.remove();
+            }
             document.querySelector(".md-header__topic").insertAdjacentHTML("beforeend", versioning);
         });